Porous film and method of making



United States Patent POROUS FILM AND METHOD OF MAKING Charles R. Kemper,50d Canterbury Drive,

Dayton, Ohio 45429 No Drawing. Continuation-impart of application Ser.No. 218,531, Aug. 22, 1%2. This application May 25, 1964,

Ser. No. 370,042

Claims. (Cl. 2602.5)

ABSTRACT OF THE DKSCLOSURE Breathable polyvinyl chloride cellular sheetsare formed from a plastisol mixture with polyvinyl chloride and aplasticizer, having dispersed therethrough particulate sodiumtetraborate decahydrate (borax) capable of passing through a 100 US.Standard mesh screen, and a water soluble cell forming material such assodium chloride. The plastisol mixture is formed into sheets andthereafter heated for release of water of hydration from the borax whichforms small channels through the sheet as the latter is being cured.These channels permit access of a leaching medium to the anhydrous boraxand water soluble cell forming material.

This invention relates to plastic film or sheets and the like, and moreparticularly it relates to a method of obtaining a flexible vaporpermeable sheet of polyvinyl chloride, and to the product of suchmethod. This invention relates more particularly to a polyvinyl chloridematerial formed by heating a plastisol to an elevated temperature ofabout 300 F. to 400 F. The plasticizer such as Union Carbide & ChemicalsCompany Flexol 380 may be used with powdered resin such as Exon 654 ofFirestone Tire & Rubber Company in about equal proportions by weight orwith the resin up to about twice the weight of the plasticizer.

Many attempts have been made to obtain a flexible porous polyvinylchloride sheet with interconnected tiny pores of fairly uniform size.Crystalline sodium chloride has been tried with polyvinyl chloridecompounds and the salt crystals later dissolved out to the extent thesolvent could find access to the crystals. However such attempts havenot been satisfactory. It is believed that due to the tenacity of thecovering of highly plasticized vinyl chloride resin surrounding theindividual salt crystals a moisture resistant barrier is present toblock out the water in the leaching process in these past attempts. Suchobjections or defects however, appear to be overcome by using borax inaccordance with the present invention.

It is therefore one of the objects of the invention to provide a novelplastic film or flexible sheet of porous polyvinyl chloride, with opencells of such character as to give vapor permeability or the ability tobreathe.

Another object of the invention is the provision of a novel method ofmaking a porous plastic sheet of this character, where the pores areprovided to a substantial degree by crystalline borax.

A further object of the invention is the method of making a flexiblesheet of polyvinyl chloride having significant tensile strength andelasticity, in which water" soluble salt crystals present in the mixturetreated are trapped or encapsulated in the plastisol during the gellingstage and before the thermo-setting or fusing temperature is obtained,and then during the heating process water of crystallization from someof the salt crystals is released and vaporized to provide open pores andform tiny intercommunicating channels between the various saltcontaining chambers.

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A further object resides in the method of making a sheet of thischaracter from a plastisol containing about 2 to 20% borax crystals.

Another object of the invention resides in the provision of a novelpolyvinyl chloride sheet or film having vapor permeability and themethod of making the same, possessing the advantageous features and themeans and modes of procedure hereinafter described.

With the above and other incidental objects in view as will be morefully apparent from the specification, the invcn'ion intended to beprotected by Letters Patent consists of the features and arrangementsand combinations as hereinafter described, or their equivalent, thepresent application being a continuation-in-part of my application Ser.No. 218,531 filed Aug. 22, 1962, now abandoned.

The type of material with which this application is concerned is apolyvinyl chloride compound. The resin may be in the form of a finepowder thoroughly mixed in a plasticizer to form a liquid suspension orplastisol mixture or dispersion. The plasticizer ordinarily employedwith resins of this type of material usually results in a mixture with agelling point at a temperature of about F. although this temperature isnot clearly defined and may have somewhat of a range especially in viewof the different kinds of plasticizing agents that may be employed.

Ordinarily, too, this type of resinous material would be cured or fusedat a temperature of about 350 F.

Attempts have been made to provide open cells of chambers of air or gasin polyvinyl chloride compounds by means of chemical blowing agents butthe past processes do not provide a satisfactory surface and are verycritical because once the gas chambers are formed by the generated gasesthere is an immediate tendency for the chambers to collapse or tocombine to form large bubbles if the chambers are in communication witheach other.

In accordance with the present invention the plastisol has added to it aquantity of water soluble salts in the form of finely divided powder.The amount of salts added is about 25% to 200% by weight of the weightof the plastisol, the latter providing a rather thick but viscousmixture. The resin may be a homopolymer or a copolymer of vinylchloride, the term polyvinyl chloride here including copolymers of vinylchloride with other vinyl resins. Exon 654 of Firestone Tire & RubberCompany has been found to be very satisfactory for this purpose. Theplasticizer employed may be one suitable for use with this type ofresin. Union Carbide and Chemicals Fiexol 380 has been found to be verysatisfactory. The proportion of resin powder to plasticizing agentshould be from about 10 to 10 to about 10 to 8 or less, theseproportions varying greatly in dependence on the equipment available andthe particular characteristics and sheet thickness desired.

The particular salt crystals employed are pore forming crystals is veryimportant. Sodium chloride crystals alone are ineffective because ifthey are thoroughly mixed they will also be thoroughly encased inresinous films during the curing stage and water cannot get access tothem later to dissolve the sait away. Glaubers salt has been tried withcertain cellulose materials since it contains some water ofcrystallization which it gives up however at a point far far below thattemperature at which a vinyl resin compound would gel and since theliberated water has a degrading effect on the plasticizer and resins ofa polyvinyl chloride character, the use of Gl-aubers salt is notsuitable. It has been found however that by using an adequate amount ofcrystalline sodium tetraborate, or borax containing water ofcrystallization as a pore forming agent, tiny increments of water areliberated from the borax during the gelling stage and prior to thefusing or setting temperature. This water liberation does not startuntil after the heating or curing stage has been started.

Sodium tetraborate decahydrate (-Na B O lH O) is the preferred salt. Thewater of crystallization is given up at a temperature well in excess of140 F. and well below the fusing temperature of the vinyl resin andvaporized at about the boiling point of water and well below the fusingor setting temperature of the polyvinyl chloride and is supplied in theform of exceedingly tiny increments from the tiny dispersed particles ofborax well distributed throughout the plastisol. The steam generatedforms tiny channels of communication between the chambers containing theborax crystals and tiny channels to the outer surfaces of the material.The time in which the water of crystallization can act on the p-lastisolis very limited, only a matter of a minute or so, so the water does nothave the degrading effect that would be produced if any substantialamount of water were present for any substantial time period.

The crystalline sodium tetrabo-rate in accordance with the presentinvention is employed with other water soluble salts, preferablycrystalline sodium chloride used with the borax in quantities forexceeding the borax present. It has been found the steam generated bythe distributed borax crystals forms passages between the chamberscontaining the borax and the chambers containing the sodium chloride sothat after curing or fusing of the resin the leaching water can haveaccess to the sodium chloride crystals to dissolve them as Well as theremaining sodium tetraborate. By using substantially more sodiumchloride than borax there is not only an economy in the material costbut in addition the sodium chloride acts as a stabilizer in that itassimilates some of the water of crystallization of the borax andprevents an undue large excess of such water as might affect thestrength of the sheet. It has also been found that by adding a percentor two of sodium bicarbonate the outer surface of the finished productis somewhat improved.

The crystals of borax after heating will remain as anhydrous sodiumtetraborate following the release of the water of crystallization, andthe sodium chloride crystals will remain in the product until the fusingor setting stage has been completed. The entire time for the heatingprocess is only about 2 or 3 minutes for thin sheets and to 7 minutesfor sheets that might be about A" thick. The temperature used is about350 F. although temperatures as high as 400 or as low as 300 arepractical. After the heating phase the sheet is carried through a waterbath where the salts are leached out. The water bath is preferablyprovided with a series of pressure rollers between which the sheetpasses to aid in the leaching process.

The relative amount of crystalline borax employed in the plastisol willvary depending upon the type of material desired but to give an airpermeable product the amount of borax should be from 2 to 20% of thecomplete mixture from which the sheet is made. The amount of sodiumchloride crystals should be about 4 to 20 times the weight of boraxused.

The following specific examples are intended as illustrations of myinvention and not as limitations thereof. For example the followingcombinations were prepared by mixing the ingredients as follows:

Flexol 380. The borax was in the form of fine powder most of which couldpass through a 100 US. standard screen. The sodium chloride crystalswere also small enough so that most of them could pass through a 100 US.standard screen. The polyvinyl chloride powder was thoroughly mixed inthe plasticizer before adding the mixed crystalline salts. The salt andborax were very thoroughly mixed with one another and with the sodiumbicarbonate, where employed, before adding to the plastisol. Roller orcrush milling of the entire compound during the mixing operation, toinsure a more complete dispersion of the crystalline materials, seems toproduce a better finished product. Entrained air bubbles are preferablyremoved by vacuum before pouring the mixture at normal temperature on atraveling carrier. This carrier may be a carrying belt of metal or othersuitable material from which the heated sheet is later released. Thesheet is then passed on the carrier through the heating zone where it issubjected to heat of about 300 F. to 400 F. for a few minutes dependingupon the thickness of the sheet. During the heating process aspreviously explained the water of crystallization in the active salt,borax, is driven off and then vaporized after the gelling phase tocreate a series of interconnecting cells in the final product. Thesecells range in size from a few microns to a few thousandths of an inch.These tiny pores are so numerous that the apparent density of theproduct is from about to 80%. The collapse of these cells producedduring the heating stage is prevented because of the salt crystalsthemselves. The hot water then leaches the soluble residues from theproduct after the curing stage is completed. The tiny interconnectingchannels between the cells extend from the surface tosurface in anincoherent manner and permits this leaching process to an effectivedegree so that substantially all the water soluble salt is quicklyremoved.

For some purposes the porous sheet may be made entirely from theplastisol but in many cases and to provide adequate strength an openweave sheet of fabric of cotton, nylon, rayon or other suitable flexiblematerial may be incorporated in the product. This fabric sheet may beapplied to the traveling belt or carrier and the resinous mixture spreadupon it or it may be added at the top of the resinous layer. It may beof advantage to finish the material as an unsupported film and thencement to the desired substrate. Where the material of the fabric is onethat is effected by high heat the curing temperature used may be about300 F.

Where the requirements are such as to require a greater proportion ofwater soluble salts with respect to plastisol it has been found to beadvantageous to add a small amount of solvent or mineral spirits to thinthe mixture before the sheet is cast and to aid in obtaining a thoroughmixture. The amount of solvent or mineral spirits may be of the order of1% by weight.

In the examples the sheets were soft to the touch and air permeable withpores throughout and in communication with both outer surfaces. Thelower sides which rested on the carrier were smooth. The upper surfaceswere rougher and somewhat tougher. When examined under a microscope thetiny pore openings were quite apparent Parts by Weight in PercentExample 1 2 3 4 Polyvinyl Chloride Besin-. 16 19 25 22 28 28 31 42 38Plasticizer 16 21 24 22 2s 26 30 35 31 Nacl 56 47 /2 44 42 35 29 10 10Borax 7 4 2% 11 5 11 9 10 18 Sodium Bicarbonate 1 1 1 3 3 In additionsmall quantities of conventional stabilizer and with no surface roughingtreatment. Example 1 was softcoloring additives were present. InExamples 2 to 9 the est to the touch and had the lowest apparentdensity, resin was Exon 654 and the plasticizer was Flexol 380. about/3. The density generally increased as the salt In Example 1 the resinwas a mixture of Exon 654 and content employed descreased. The sampleshaving the Monstantos 440, and the plasticizer was a mixture of leastcapability of vapor transmission were the ones resultknown plasticizingagents including the aforementioned ing from the smaller content ofwater soluble salts. The

sample resulting from the use of a minimum of sodium chloride were ofhigher apparent density.

It has been found that the salt crystals can be mixed in the plastisolfor a long time prior to the time of use. It is not necessary to mix theingredients and use them immediately as it is in the case of someblowing agents and organasols. The resulting product is stable and thetemperatures of treatment and proportions are not exceedingly criticalso the process is capable of being very conveniently employed. Within awide range of porosity the sheet may be of a desired apparent densitymerely by choosing the right water soluble salt content for the purposedesired. The resultant product is one that is'strong, stable, odorless,flexible and is capable of breathing. It is not a water barrier since itis to some degree water permeable but only water permeable to a degree.Water would travel slowly through all of the examples referred to. Thisproduct may thus be used where a water repellant characteristics isdesired but since it has the capability to breathe, since the poresprovided form channels clear from one side of the sheet to the other,the material is particularly satisfactory for use in many places wherethis characteristic is an advantage, for example, in clothing, shoes,upholstery, carpeting, and many other uses.

While the invention has been discussed at length it should be understoodthat the invention is not limited to the specific features andproportions referred to, but that the method herein disclosed comprisesbut one of several modes of putting the invention into effect.

Having thus described my invention I claim:

1. The method of producing a flexible vapor permeable polyvinyl chloridesheet material with interconnecting pores comprising the steps offorming a plastisol mixture comprising polyvinyl chloride resin, aplasticizer and a particulate water soluble crystalline materialuniformly dispersed in said plastisol, said water soluble crystallinematerial comprising sodium tetraborate and a particulate water solublematerial different from said sodium tetraborate, said sodium tetraboratecontaining water of hydration releasable at a predetermined temperature,forming said plastisol mixture into sheet material, advancing said sheetmaterial into a heating zone having a temperature sufiiciently high torelease said water of hydration and to effect curing of said plastisol,maintaining said formed plastisol sheet material in said heating zonefor a period of time sufficient to effect release of said water ofhydration and to cure said plastisol, the release of said water ofhydration providing tiny interconnecting channels randomly extendingtherefrom one surface to the other of said formed sheet material, andthereafter treating said cured sheet material with a leaching medium todissolve the remaining soluble residues in said formed sheet material toprovide a cured sheet having pores and interconnecting channels therein.

2. The method as set forth in claim 1 wherein said last treating stepincludes passing said cured sheet material through a series of pressurerolls.

'3. The method as set forth in claim 1 wherein said sheet member isformed on a travelling carrier member.

4. The method as set forth in claim 1 wherein said plastisol mixture isformed by separately mixing said polyvinyl chloride resin andplasticizer, and said water soluble material and said sodiumtetraborate, and adding said mixture of water soluble material and saidsodium tetraborate to said mixture of polyvinyl chloride resin andplasticizer.

5. The method as set forth in claim 1 wherein said water solublematerial different from sodium tetraborate is sodium chloride, and saidparticulate water soluble material being present in an amount by weightbetween 25% and 200% the weight of said plastisol.

6. The method as set forth in claim 1 wherein said formed sheet materialis heated to a temperature of between 300 F. and 400 F.

7. The method [as set forth in claim 1 wherein said sodium tetraborateis present in an amount by weight of between 2% and 20%, and said watersoluble material different from said sodium tetraborate being sodiumchloride present in an amount by weight of between 4 and 20 times theweight of said sodium tetraborate.

8. The method as set forth in claim 7 wherein said plastisol mixturecontains between 1% and 3% by weight of sodium bicarbonate.

9. The method as set forth in claim 1 wherein said plastisol mixturecontains an organic solvent.

10. The method as set forth in claim 1 further including the step ofremoving entrained air from said plastisol mixture prior to formation ofsaid plastisol mixture into a sheet material.

11. The method as set forth in claim 1 further including the step offorming said plastisol sheet material on a fabric base prior to curingthe polyvinyl chloride resin and plasticizer.

12. The method as set forth in claim 1 wherein said cured and leachedporous sheet is adhered to a substrate.

13. The method as set forth in claim '1 wherein said formed plastisolsheet is heated for a period of between 2 to 7 minutes.

14. The method as set forth in claim 1 wherein said plastisol mixture ismilled prior to formation of said plastisol sheet.

15. The method as set forth in claim 1 wherein said water solublematerial different from sodium tetraborate is sodium chloride, saidpolyvinyl chloride resin being present between 16 and 42 parts byweight, said plasticizer being present between 16 to 35 parts by weight,said sodium chloride being present between 10 and 60 parts by weight,and said sodium tetraborate being present between 2 /2 to 18 parts byweight.

References Cited UNITED STATES PATENTS 2,256,483 9/1941 Johnston 260-252,911,382 11/1959 Barkhuff et al. 260-25 2,984,869 5/1961 Honey et al.2602.5 3,055,966 9/ 1962 Sundberg 2602.5

FOREIGN PATENTS 649,646 1/ 1951 Great Britain.

MURRAY TILLMAN, Primary Examiner.

M. FOELAK, Assistant Examiner.

